• Before reading this article, please make sure you master PN junction 、 Detailed explanation and application of diode principle The content in
• For the symbol of triode 、PNP and NPN The basic knowledge such as the difference between is not explained here
  

  List of articles

  • 1 principle
  • 2 characteristic
  • • 2.1 electric current
    • 2.2 Input features ：I_B And U_BE
    • 2.3 output characteristic ：I_c And U_CE
    • 2.4 Zoom in 、 saturated 、 end

1 principle

Start with NPN Take triode as an example ：

• The collector electrons in the figure should be painted less ( Less doping ), The size should be drawn higher ( Large size , Not easy to heat ), It will be explained later

The base and emitter are positively biased , So the electrons from the emitter will spread to the base . Due to collector and base PN The knot is reversed , Its depletion region increases , Direction is N->P The electric field increases . At this time, increasing the voltage is not like doping with normal concentration PN The drift current is almost constant with the increase of voltage , Because the doping of the base is very little and very thin , So the electrons that diffuse to the base have little time to combine with the holes in the base ( Only a few are combined ), Most electrons can easily reach the top and reverse bias PN The internal electric field of the junction enters the collector .
Process manufacturing ：
（1） The base ： Must be very thin 、 The doping concentration is very low
（2） The emitter ： Inject electrons into the base , So it needs a lot of doping
（3） Collector ： Strictly speaking , No matter how much the collector is doped , But we don't want to C-B Of PN The junction is broken down , Therefore, it is necessary to ensure a large breakdown voltage . Recall the zener diode , Its doping is high , So the breakdown voltage is small . Now the base doping is very little , So the less doped the collector , The greater the breakdown voltage . Therefore, the doping of collector should be less .

• In some transistors, the doping concentration of collector is between the base and emitter , In some transistors, the collector is less doped than the base , So there are no hard and fast rules

（4） Size ： The collector size should be larger than the base , In this way, when the current flows , It won't get hot so easily ; The base is highly doped , So it can't be too big .

So the actual triode may be like this ：

2 characteristic

Take the common emitter amplifier as an example

2.1 electric current

The electrons coming out of the base and the electrons coming out of the collector will eventually return to the emitter (B、E、C Finally, they are grounded ), As an injection of electrons . namely I_E=I_B+I_C.

2.2 Input features ：I_B And U_BE

• When U_BE<0.7V when ,B and C Very little current
• U_BE>0.7V when ,I_B a surge , however I_B be relative to I_C It's still very small .
• If you increase U_CE, Above PN The depletion region of the junction increases , be U_BE It needs to be bigger ,I_B Will start to surge . But this change is small , So you can say U_CE No effect U_BE Size .
  

2.3 output characteristic ：I_c And U_CE

When U_C The value of is less than 0.7V when , above PN The knot is positive and partial . The assumption is 0.3V,U_BC=0.7-0.3V=0.4V, The width of depletion region decreases ,C Polar electrons may spread to B extremely , and E Polar diffusion to B Polar electrons , It will still spread to C extremely , But it's not as easy as it used to be . So at this time I_C Will follow U_CE To reduce by .

2.4 Zoom in 、 saturated 、 end

As shown in the figure , Suppose the transistor amplification factor is 100 times ,C Pole connection 3V Then one 1K resistance , here C Maximum current of pole I_Cmax=3mA.
（1） Cut off zone ： When U_BE＜0.7V Time is up , here I_B≈I_C=0,C The pole resistance has no voltage drop , therefore U_CE To the maximum 3V
（2） Zoom in ：U_BE≈0.7V And β*I_B<I_Cmax, As mentioned earlier, this pressure drop is almost unaffected by U_CE influence , about Si It's usually 0.7V. here ,0V<U_CE<3V.
（3） Saturation zone ：U_BE≈0.7V And β*I_B>I_Cmax, because C The polar current cannot be higher than 3mA, therefore I_C Keep at maximum 3mA Can't rise any higher ,U_CE=0.

Switch action ：

• In the saturation zone ,C Pole current max 、U_CE=0, amount to CE Pole short circuit
• When the cut-off zone ,C There is no current at all 、U_CE To the maximum , amount to CE Pole open circuit